CIRCULATORY SYSTEM IN ARACHNIDA 183 



In his work on the organs of circulation in Schizopoda, pub- 

 hshed in 1883, Delage writes (I translate the original): ''To 

 determine exactly the length of the heart one should first of all 

 well define its limits. It happens that these limits are not easily 

 traceable because the diameter of the heart is not greatly differ- 

 ent from that of the aortae at their points of origin. (Delage has 

 in mind both the anterior and posterior aorta.) They are marked 

 by the presence of cardioaortic valvules which have not yet been 

 described by anyone. Moreover, these valvules are identical 

 with those which are found in the same place in Amphipoda." 

 "Within these limits the heart extends from the level of the last 

 maxillary segment to the superior portion of the last thoracic 

 segment." The anterior aorta, the median stem of which ends 

 in the upper lip, gives off four branches in its course : the common 

 trunk of the ophthalmic arteries, the cerebral artery, and the two 

 antennal arteries. It may be of interest to notice that the 

 sternal artery in Schizopoda arises from the heart. 



The structure of the aortic valves themselves is different from 

 those of Arachnida. They are paired lateral structures, as in 

 all other Crustacea. The position indicated by Delage, taking 

 into account evidence derived from the study of all other organs, 

 is two somites nearer the head than in Arachnida. In such 

 Decapoda as the crayfish and the lobster the heart is distinctly 

 limited and considerably modified. Instead of arising from the 

 aorta, the antennal arteries arise directly from the heart and 

 have their own valves. Yet the aorta has also valves at its 

 base and these are of the same type as in Schizopoda and 

 Amphipoda. The position of these valves coincides exactly with 

 the semilunar sulcus of the carapace, the two ends of which open 

 into the so-called cervical groove. There are therefore differences 

 in regard to structure of the circulatory system in closely related 

 orders of the same sub-class — differences which cannot be 

 understood without special study directed to their elucidation. 



We know still less of the Protracheata, Pycnogonida, and 

 the four classes formerly comprised under the general name of 

 Myriapoda. Although I have some investigations under way, 

 I am not prepared as yet to make any definite statement. 



